Magnetometer gradiometer apparatus and method



Dec. 2, 1952 Filed May 27, 1947 F. M. MAYES ETAL 2,620,381

MAGNETOMETER GRADIOMETER APPARATUS AND METHOD 6 Sheets-Sheet l IJLi-l.

AMPLIFIER 8 RECTIFIER OSCILLATOR PLIFIER DREOTIFIER OSCILLATOR FIG; 2%

AMPLIFIER BRECTIFIER QIVUC/YM/ 1 7M Mayes A. J Tickner Dec. 2, 1952 F. M. MAYES EI'AL 2,620,381

MAGNETOMETER GRADIOMETER APPARATUS AND METHOD Filed May 27, 194'? 6 Sheets-Sheet 2 220 V. 32 263 E I I I I I'I I I'I I I 26 354 wuowtow FMMayes A.J Tickner Dec. 2, 1952 F. M. MAYES EI'AL MAGNETOMETER GRADIOMETER APPARATUS AND METHOD Filed May 27, 1947 6 Sheets-Sheet I5 o I wcmm A.J Tiehgz r IIII IIII mm in Q8 om 08 mm 5m 5m mmm mmm wmm 5N hhm 9 EN mmm mmm .Om mOn mom mom Non NM vOm :m Ohm wm wig Dec. 2, 1952 F. M. MAYES ETAL MAGNETOMETER GRADIOMETER APPARATUS AND METHOD 6 Sheets-Sheet 4 Filed May 2'7, 194'? DISTANCE-FEET DISTANCE FEET Dec. 2, 1952 F. M. MAYES ETAL 2,620,381

. MAGNETOMETER GRADIOMEZTER APPARATUS AND METHOD Filed May 27, 1947 6 Sheets-Sheet 6 AMPLIFIER Lt l RECTIFIER AMPLIFIER I GREGTIFIER BREGTIFIER IO I5 0 l 1 WL I OSCILLATOR -2' OSOILLAM AMPLIFIER IREGTIFIER 6| Patented Dec. 2, 1952 MAGNETOMETER APPARATUS AND METHOD Fred M. Mayes, Ridley Park, Pa., and Alvin J. Tickncr, Hollywood, Calif.

Application May 27, 1947, Serial No. 750,865

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G. 757) 25 Claims.

This invention relates to a new and improved method and apparatus for measuring the gradient of a magnetic field, in which a pair of field detectors spaced apart a predetermined distance within the field serve as gradient sensitive detectors and are employed to derive signals differing in magnitude proportional to the gradient of the field. More specifically, the invention contemplates the use of a third field detector spaced between the two gradient sensitive detectors and responsive to the same directional component of the magnetic field to which the end detectors are responsive. The signal derived from the centrally located detector is amplified and employed to generate electromagnetic fields at at least the two outer detectors opposing the field to be measured, to reduce the background of steady field and to enhance the relative difference in field magnitudes at the two gradient sensitive detectors in relation to the steady field, thereby providing field stabilization. Apparatus is energized from the two outer detectors responsive to the difference in outputs thereof, and has associated therewith a device for indicating the difference and direction of the gradient of the field.

In another embodiment of the invention, a third opposing field is generated at the centrally located detector by the same current which generates the fields at the outer detectors. This third generated field is automatically maintained substantially equal in magnitude to the ambient field to be measured at the center detector, thereby providing maximum stability for the ambient field stabilizing.

Means is provided for measuring the current setting up the opposing fields, thereby to provide a measurement of the average value of the ambient field when the current is proportional to the strength of the ambient field at the centrally located detector.

The use, with gradiometers having two magnetometer elements spaced apart a predetermined distance, of additional coils associated with the magnetometer elements for producing fields opposing the field to be measured, is old in the art, being shown in Patent No. 2,379,716 to Albert W. Hull, granted July 3, 1945, and elsewhere. In these prior art devices, the opposing or nulling fields were usualiy manually adjusted, usually by a rheostat in series with the nulling coil and a source of power. Adjustment was frequently slow and critical, and manual readjustment was necessary for each change in the average value of the field to be measured. The necessity for the continual, slow readjustments as the aver age field changes placed severe limitations upon GRADIOMETER the usefulness of the devices in question. Furthermore, if the ambient field were reversed, the current through the nulling coil had to be reversed and readjusted in value.

The preferred embodiment of the invention described and claimed herein overcomes the above mentioned disadvantages by the use of a third magnetometer element to derive a signal proportional to the intensity of the average value of the gradient field and having a characteristic determined by the polarity of the field. This signal is then amplified, rectified, and applied to field generating coils disposed at all three magnetometers thereby to generate fields opposing the field of which the gradient is to be measured. The magnitude and polarity of the generated fields are automatically maintained at the desired values. The result is that the average field may be substantially reduced to zero and only the difference or gradient remains at the gradient detectors. This, as will be subsequently apparent, results in tremendous improvements in stability and accuracy of measurement, and in marked simplification of the electrical apparatus required.

The subject invention also presents improvements, as will be hereafter apparent, over other patents in the gradiometer art, for example, Patents Nos. 2,406,870 and 2,407,202, issued Septemher 3, 1946, to V. V. Vacquier for Apparatus for Responding to Magnetic Fields. The Vacquier patents do not disclose means for automatically maintaining a nulling field of desired intensity and polarity. Patent No. 2,252,059, issued August 12, 1941, to G. Earth for Method and a Device for Determining the Magnitudes of Magnetic Fields does not provide means for automatically determining the polarity of the field generated by the device which provides electromagnetic feedback therein. Patent No. 2,053,154, issued September 1, 1936, to C. W. La Pierre for Direct Current Indicator, provides a bridge detector circuit but is not concerned with nulling or field stabilization.

One of the objects of the invention is to provide a new and improved method of measuring the gradient of a magnetic field.

Another object is to provide a field gradiometer in which an added magnetometer is used to derive a signal indicative of the magnitude and direction of the average value of the field and to utilize this signal to stabilize the gradient reading against spurious signals due to fluctuations in the average value of the ambient field.

Another object is to provide a magnetic field gradiometer in which a nulling field is generated automatically, and continually maintained automatically with a direction and magnitude to oppose the average ambient held in a predetermined amount.

Another object is to provide a new and improved detector circuit for use with a gradioineter employing a pair of fiuxgate magnetometer elements.

Still another object is to provide a gradiometer having ambient field stabilization, in which the average value of the field at the gradiometer detectors is largely eliminated, thereby increasing the difference in field strength in relation to the average strength of the remaining field, and accordingly increasing the ease of measurement of the gradient. 7

A further object is to provide a new and improved magnetometer gradiometer in which asymmetries in fundamental frequency signal outputs due to the presence of even harmonics are utilized to provide measurements of the strength and gradient of an external magnetic field.

A further object resides in the provision of a magnetometer gradiometer employing a plurality of magnetometer elements arranged in bridge circuits responsive respectively to the strength and gradient of an external magnetic field.

Still a further object of the invention is to provide a new and improved structure for maintaining a plurality of magnetometer elements comprising a magnetometer gradiometer system in predetermined physical and magnetic alignment with respect to each other.

Other objects, improvements, and advantages not specifically set forth hereinbefore will be apparent after a consideration of the specification and drawings in which:

Fig. 1 is a diagrammatic view, partially in block form, of the preferred embodiment of the invention;

Fig. 2 is a diagram in block form of a generalization of the embodiment of Fig. 1;

Fig. 3 is a diagrammatic view of the complete electric system of the magnetometer gradiometer forming the preferred embodiment of the invention;

Fig. 4 is a view in section of a supporting assembly and coil arrangement suitable for use with the circuit of Fig. 3;

Fig. 5 is a block diagram of an alternative physical arrangement of the detector elements of Fig. 4;

Figs. 6, 7, and 8 are curves illustrating the operation of the magnetometer gradiometer of Fig. 3;

Figs. 9-, l0, and 11 are diagrams illustrating the operation of the apparatus of Fig. 3;

Figs. 12, 13, and 14 are curves further il1ustrating the operation of the apparatus of Fig. 3;

Fig. 15 is a circuit diagram, partially in block form, of a magnetometer gradiometer employing no field stabilization;

Fig. 16 is a circuit diagram illustrating an alternative form of the apparatus of Fig. 1;

Fig. 17 is a circuit diagram of a second embodiment of the invention; and

Fig. 18 is a circuit diagram illustrating an alternative form of the apparatus of Fig. 17.

Referring now to the drawings in which like numerals are used throughout to identify like parts, and more particularly to Figs. 1 and 3 thereof, there is shown at l2 and 13 two similar and balanced saturable magnetic cores disposed in symmetrical mutual relationship and spaced apart a predetermined small distance with their axes parallel to each other and to the component of the magnetic field which it is desired to measure. About these coils are disposed similar and balanced coil windings l5 and I? respectively. The coils are connected in series and to the ends of the center-tapped secondary 46 of a transformer generally designated at l i and having primary 45, the primary being connected to an oscillator or source of power 2. The source 2 is designed to supply sufficient voltage and current of substantially sinusoidal waveform to energize coils l8 and H sufficient to drive cores i2 and 13 well into saturation during a portion of each half cycle, and the transformer til may be particularly designed to introduce substantially no distortion into the waveform of the current. The center tap between coils is connected to one end of the primary 52 of transformer 5| which may also be designed to deliver an output which is substantially identical in waveform to the input thereof, the other end of the primary 52 being connected to the center tap of secondary 36. These connections effectively provide a bridge circuit in which the four arms comprise the coils It and H and the two halves of secondary The output of the bridge is delivered to the aforementioned primary 52.

A schematic diagram of a circuit suitable for use with the oscillator generally designated by the numeral 2 in Fig. 1, is shown in Fig. 3, and includes the components associated with tube 26. Whereas any suitable tube may be used, tube 23 is shown as a double triode having common cathodes 3 i, grids 2i and 2S, and anodes or plates 29 and 3%) respectively. Plate 39 is coupled through condenser 33 to grid 27, and plate 253 is coupled through condenser 3d to grid 28. Return to ground and cathode from grid 2'5 is through choke 23, and return to ground and cathode from grid 28 is through choke 2 3, the grids having a condenser 25 connected thereacross to assist in shaping of the oscillator waveform. Between the anodes or plates 23 and 3b is connected a center-tapped inductance 3?, having a decoupling resistance 35 in the lead 32 between the center tap and the plate supply battery H37, and having bypass condenser 35 from the center tap to ground. When component values are suitably chosen, the circuit so traced constitutes a negative resistance oscillator, characterized by a symmetrical waveform output, the operation of which will be readily understood by those skilled in the art, being expounded in Henney: Radio Engineering Handbook, third edition, page 303, and elsewhere. The voltage developed across inductance 37 is further shaped by the L-C network comprising condenser 39 and choke 35 in parallel, and is fed to the primary d2 of a transformer generally designated at il, the transformer having condenser s!) across the input thereof to assist in maintaining the most symmetrical waveshape. Any suitable frequency of oscillation may be employed, so long as it does not have low order harmonics corresponding to the low order harmonics of a second oscillator to be subsequently described. In the embodiment shown a frequency of 600 cycles is generated in the oscillator circuit of tube 26. The secondary Q3 of transformer El is connected to and excites the aforementioned primary 45.

Assume now by way of description that the terminals of coils l6 and I! are connected for measurement of the component of the field at cores l2 and I3, and that the arms of the bridge are symmetrical, and recalling that the current from source 2 drives the cores to saturation, harmonics are introduced into the waveform of the exciting voltage. The manner in which distortions due to harmonics are introduced in the sinusoidal waveform of an alternating current voltage which is exciting an inductance having a ferromagnetic core is well known, but may be reviewed briefly here to form a basis for a description of the invention to follow. A suitable material for use in the cores l2 and [3 of the subject invention is known in the trade as Permalloy, the 3-H curve of which is characterized by a small area indicating the low loss characteristics of the material. Fig. 12 shows the BH curve of the ferromagnetic core of a coil when excited by a sinusoidal voltage Q. The back voltage, or voltage due to the inductance of the coil, is represented by the curve N. The peaks in voltage N occur when the rate of change of the flux, as determined by the rate of change of the exciting current and the rate of change of the slope of the B-H curve, is

maximum. If no external field is acting on the core, and if the core material has low retentivity, the positive and negative alternations of voltage N will be symmetrical. Assuming certain relationships between the impedance of the coil and the impedance of the exciting source, odd harmonics will be present in substantial amounts in the exciting voltage and current.

Assume now by way of description that a given magnetic bias or polarizing field is applied to the ferromagnetic core. Several effects may be noted. The core saturates sooner on one half cycle of the exciting voltage, when the polarizing field aids the magnetizing force generated by the exciting current, than it does on the other alternation when the polarizing field opposes the magnetizing force of the exciting current. The B-H curve may be thought of as shifted to the right or left, Fig. 12, depending upon the direction of the polarizing field, by amounts represented for example by lines is or Z. In actuality, the shape of the B-H curve may change somewhat, the rates of change of the slopes thereof becoming unsymmetrical on the two sides.

Assume by way of explanation that the core having the B-H curve of Fig. 12 is disposed within a magnetic field of a polarity such that the vertical zero axis (B axis) is shifted to the right in the figure, and becomes the line it. The exciting voltage may now be represented as having its zero axis shifted to the right also, and may be represented by the voltage Q. The instant of the maximum rate of change of the flux is advanced in time from its former position as the right side of the B-H curve is traversed, and retarded in time from its former position on the other side of the 3-H curve. As a result, the alternations of the induced volt-age are retarded and advanced selectively, as illustrated by curve N in Fig. 12. Changes occur also in the relative magnitudes of the peaks of the alternations of voltage N, since the peak rate of change of the flux in the circuit is increased on one side of the B-H curve and decreased on the other. One alternation of curve N, for example the upper alternation in Fig. 12, is increased in peak ampltiude as well as being advanced in time, while the lower alternation of the voltage of curve N is retarded in 6 time and correspondingly decreased in amplitude. The curves of Fig. 12 are not intended to be precise studies of the waveforms, but are exaggerated to be more illustrative of certain amplitude and time relations in the operation of the device. Whereas an amplitude change in the voltage is indicated by the Figure 12, the effective phase shift produced by the bias field is the primary source of the useful signal output of the exciting coil of the core in the presence of the external field.

Assume now by way of description that the axial or polarizing field along the core is reversed in polarity, and the zero vertical axis is shifted to the left in Fig. 12 to line I. The exciting voltage may be indicated by the waveform Q, and the induced voltage by the waveform N. It is observed that the alternation of the induced voltage which was before increased in amplitude and advanced in time is now decreased in amplitude and retarded in time, while the alternation which was before decreased in amplitude and retarded in time is now increased in amplitude and advanced in time.

The presence of asymmetries in the waveforms of N and N", as evidenced by the fact that the alternations are not mirror images of each other, indicate the presence of even harmonic components therein, which are introduced when the core has a polarizing force or steady axial field component acting thereon. It can be shown mathematically or graphically that these harmonics are generated in amounts proportional to the strength of the axial field, or polarizing field, on the ferromagnetic core.

Assume now by way of description that the coils l6 and IT are energized and that no steady axial magnetic field is present in the cores. The waveform of the induced voltage in each coil is symmetrical, the curve R of Fig. 13 illustrating the voltage in coil 16, and the curve U illustrating the voltage in coil l1. Actually, of course, these voltages are in phase in the two coils since the coils are in series, but, with reference to the output or detector circuit of the bridge, they are in phase opposition, as is well known in the art, so that the voltages of Fig. 13 are in phase opposition with respect to the primary 52. Since they are equal and opposite, if the bridge is perfectly balanced, they will add algebraically to zero and there will be no signal output, and no excitation of primary 52.

Perfect balance of the bridge, while theoretically possible, may be extremely difficult of achievement under actual operating conditions, for a variety of reasons. For purposes of description, it may be helpful to divide balance problems and considerations into two groups. The first will be referred. to herein as fundamental balance. Fundamental balance requires that the transformer tap be at true center, and that the resistance, inductance (including mutual and reflected inductance), distributed capacity, etc., be matched for all odd harmonics at all times throughout a time cycle. Actually, while fundamental balance, as defined, cannot be realized in practice, it can be satisfactorily approximated by a careful choice of the transformers 44 and 4!, and by empirical matching and balancing of the two coils and cores, as will be hereafter elaborated. A second group of balance considerations may be treated under signal balance, which is achieved if there is zero axial external field, zero permanent magnetization of the cores, and zero amount of even harmonics in the exciting power supply. It is apparent that all of these requirements for perfect fundamental and signal balances will be impossible of achievement; as a result, the output of the bridge of coils l6 and il will not be zero in the absence of an external field, but will approximate that indicated by the waveform of Fig. 6.

Fig. 13 also shows the voltage waveforms in coils H and El when in the presence of a steady field. As previously stated, whether the steady field aids or opposes the magnetizing force during a particular half cycle of the exciting voltage depends upon the direction of the field and the instantaneous polarity of the voltage. It will accordingly be understood that the coils of the bridge may be connected so that the asymmetries introduced in the voltage waveforms, or the even harmonics generated therein, oppose or add with respect to the detector or bridge output circuit. Fig. 33 illustrates the condition in which the outputs are in addition, that is, the peak of one half cycle is advanced in time and increased in magnitude while the peak of the half cycle of opposite polarity (in the bridge output circuit) is retarded in time and decreased. in relative magnitude. Such an arrangement can be obtained under two sets of conditions. Either the coils are wound in the same direction and the steady fields acting on the cores thereof are of opposite polarity, or the coils are wound in opposite directions and the fields are of the same polarity. Since, as previously explained, coils i5 and il re adapted to be disposed side by side in a magnetic field, the axial component along cores i2 and 13 must be in the same direction, so that to obtain the conditions of Fig. 13, the coils are wound in opposite directions (or the terminals of one cell reversed). in the Fig. 13, lines ,1] establish a time reference at which both pealrs would occur simultaneously in the absence of an external field. Because of the presence of a field, which adds to the magnetizing force on the upper half cycle, voltage Waveform R. is displaced to form curve B, which has a greater peak amplitude by the distance s, and is set forward by the time q. At the same time the negative half cycle has the field opposition to the magnetizing force, hence curve 'U is reduced in peak amplitude by an amount '6, and the occurrence of the peak is retarded a time T. On the next alter- :tion, voltage U has reversed in polarity and L s peak is magnified and advanced while R" has also reversed in polarity and is accordingly reduced in magnitude and its peak retarded in time.

When the voltages R and U of 13 are added algebraically to obtain the detector output voltage, it is obvious that the output voltage will be asymmetrical about its zero axis, and it will be understood by those skilled in the art that the waveform will have a major component equal to twice the fundamental frequency, as well as having other even order harmonic components. It is of course understood that curves R, R, U,

U ar approximations devised for illustrative purp see. The bridge detector output voltuncler the magnetic field conditions described resemble somewhat the waveform of the curve of Fig. 7.

Should the ambient field at coils l5 and il be reversed in direction or polarity, the alternations of negative polarity of the waveforms of Fig. 13 would achieve the greater peak amplitudes;

8 hence, the detector or bridge output would be expected to resemble the curve of Fig. 8.

It will be apparent then that the detector bridge of coils l6 and I? delivers a signal to primary 532, Fig. l, which has characteristics representative of both the intensity and polarity of the axial component of the magnetic field to be measured; the alternations of the bridge output are of unequal amplitudes in amounts proportional to twice the intensity of the field, and selectively in accordance with the direction of the field. It is further recalled that a reversal of the direction of the steady field causes a reversal of the asymmetries of the curves of Figs. 7 and 8 with respect to the zero axis; that is, when the field reverses, the half cycles which were of greator amplitude become of lesser amplitude.

Referring again to Fig. 1, the secondary 53 of transformer 5| delivers its output to an amplifier generally designated by the reference numeral i. Whereas other amplifier circuits may be employed, in the amplifier l, in the preferred embodiment of the device, a differential peak amplifier circuit somewhat similar to that disclosed in an application by Alvin J Tickner. for Flux Measuring System, Serial No. 475,760, filed February 13, 1943, is employed. Particular reference is made now to Fig. 3 which contains a complete schematic diagram of a differential peak amplifier circuit suitable for use therein, comprising electron discharge tubes 58, 15, and 94, and associated parts. The aforementioned input transformer 5i has the aforesaid primary winding 52 with the condenser 5G connected thereacross, and has a center-tapped secondary 53 having resistances 55 and 56 across the ends thereof to the center tap, the center tap being grounded and connected through biasing resistor 5i to the cathodes 6i and 62 of the tube 53. Whereas any suitable tube may be employed, the tube shown is a double triode having, in addition to the aforementioned cathodes 6i and 62, grids 63 and E i respectively, and anodes 65 and 66 respectively. The ends of the input transformer secondary 53 are connected to the grids respectively of the tube; the anodes are connected to the ends of the center-tapped primary 12 of a coupling transformer l I, the center tap of the primary going through lead H38 to a source of plate potential Ill? and having in series therewith the decoupling resistance [06 having by-pass condenser I05 connected thereacross to ground. The operation of the amplifier circuit of tube 58 is conventional for push-pull transformer coupled amplifiers and need not be described in detail. It is noted that the absence of a by-pass condensor across bias resistance 57 provides for some degeneration in the circuit, thereby offering an improved response characteristic.

The coupling transformer "H has double matched secondaries l3 and l t, one end of each of the secondaries as shown being connected to the anodes l8 and ll of a double diode tube 75 having cathodes l8 and 19 respectively. The other ends of the secondaries l3 and 14 are connected through resistances 8! and 88 respectively to the tube cathodes l8 and i9, the resistances being paralleled by condensers 83 and 84 respectively. Th circuit thus described constitutes a dual rectifier in which a rectified voltage having a value corresponding to the peak value of the A. C. voltage of one half cycle with respect to the zero axis is developed across resistance 87, while a rectified voltage having a value corresponding to the peak value of the A. C. voltage of the other half cycle is developed across resistance 88. In that half of the cycle during which the A. C. voltage is increasing and of polarity such that diode section 16-18 is conducting, condenser 83 charges rapidly through the relatively low impedence of secondary 13 and through diode section 19-18 whereby the voltage developed across condenser 83 and resistance 81 is nearly equal to the peak value of this half cycle of the A. C. voltage. When the A. C. voltage begins to decrease from the peak value thereof, condenser 83 remains charged to nearly the peak value for the reason that it cannot discharge back through the rectifier section 16-43 and discharges slowly through the relatively high resistance of resistance 81. The time constant BC -of condenser 83 and resistance 81 is chosen to be large enough so that the condenser discharges very little between peaks, but small enough so that rapid changes in the peak values of alternate half cycles can be indicated. Similarly, in that half of the cycle during which the A. C. voltag is increasing and of polarity such that diode section '|119 is conducting, condenser 84 charges rapidly to develop a voltage thereacross which is nearly equal to the peak value of this half of the cycle of the A. C. voltage. When the A. C. voltage begins to decrease from the peak value, condenser 84 remains charged to nearly the peak value for the reason that it cannot discharge back through the rectifier section 11-49 and discharges slowly through the relatively high resistance of resistance 88. The time constant RC of condenser 84 and resistanc 88 is substantially identical to that of condenser 83 and resistance 81.

The voltages developed across condensers 83 and 84 are applied to the grids 91 and 98 of tube 94. Whereas any suitable tube may be employed, the tube 94 is shown as a double triode having cathodes 95 and 96, grids 91 and 98 respectively, and anodes t9 and led respectively. The resistance 9| connected between both cathodes 95 and as and ground provides for a steady component of bias. The grids 91 and 98 are connected by resistors 89 and 9|} in series, the common point between resistors being connected to ground. The condenser 86 of relatively large value is connected between the grids 91 and 98 to provide a suitable time constant for the circuit. The anodes Q9 and we are connected to plate load resistors i533 and H14 respectively, and thence together through the aforementioned decoupling resistor iefi to the source of plate potential lei. The circuit thus traced constitutes a difierential peak amplifier. The rectified voltages across resistance 81 and 88 are both applied simultaneously to the resistance network comprising resistances 89 and 99 between the grids 91 and st of tube at. The voltage across resistance 31 tends to make grid 91 more negative, whereas the voltage across resistance 88 tends to make grid 98 more negative with respect to cathode. When these voltages are equal, they cancel and no resultant voltage is applied to the grids. When one of the rectified voltages exceeds the other, for example, the voltage across resistance 81 exceeds that across resistance as, grid 91 is made more negative and grid 98 more positive. Anode 99 passes less current and anode I08 more corrent. The anodes assume different potentials, and this causes a D. C. current to fiow through the utilization circuit connected across the anodes, in this case the field generating coils 20, 22, and 2| to be described more fully subsequently. The inductances HH and H12 are included in the leads to the anodes 98 and I00 respectively to prevent large components of the alternating current frequency from reaching the field generating coils. The utilization circuit including the coils 29, 22, and 2| has connected in series therewith the current indicating device 5, which may be of the type in which zero reading occurs in the center of the scale.

The field generating coils 2D, 22, and 2| are disposed in predetermined spaced relations to gradiometer coil I 3, magnetometer coils I6 and i1, and gradiometer coil l5 respectively, the gradiometer coils to be subsequently more fully described, and coils 20, 22, and 2| are adapted when energized to generate electromagnetic fields having the main components thereof in alignment with the component of the field to be measured. In one application or mode of operation of the invention, coils 20 and 2| serve as nulling or field stabilizing coils and generate fields opposing the field to be measured, thereby to reduce the average field and enhance the gradient. In another mode of operation of the invention, coils 20 and 2| may be thought of as neutralizing coils, when they generate fields substantially equal in magnitude to the average value of the field.

The leads I59 and ISO from the amplifier to the field generating coils have connected in circuit therewith a double throw switch 200. A resistance |e8 is connected in series with a variable resistance ltiS across the center terminals of switch 289. When the switch is closed in the left hand position, Fig. 3, these resistances are connected across the coil 2|); when the switch is closed in the right hand position, the resistances are connected across the coil 2|. These resistances provide means for balancing the outputs of the two coils 2e and 2| if desired, the output or" one of the coils being adapted to be made slightly less than that of the other in adjustable amounts depending upon the position of the switch 289 and the variable resistor I99. Coil 22 as previously mentioned is disposed in symmetrical relation to coils l6 and H and, when energized, sets up a magnetic field which is parallel to the magnetic axes of cores l2 and I3 and, therefore, is in alignment with that component of the external ambient field to be measured.

It will be recalled that the signal input to the amplifier i has the characteristics of either the curve of Fig. 7 or that of Fig. 8 depending upon the polarity of the field along the axes of cores I2 and I3 of coils |6 and I1 respectively. It is also recalled that these peak voltages are amplified in tube 58, rectified in tube 15, and applied to the dual vacuum tube arrangement of tube 94, where the direct currents flowing through the anodes 99 and H39 are reciprocally increased and decreased selectively in accordance with the direction of the field and in amounts corresponding to the difference of the peaks of the alternate half cycles of the signal voltage. It will be obvious that connections for coils 20, 22, and 2| may be made which permit the fields set up thereby to oppose the field to be measured. It is obvious also that should the field to be measured reverse in polarity, the generated field in the coil 22 will be automatically reversed in polarity. When sufficient amplification is available in amplifier the field generated by coil 22 is substantially equal to the ambient field at coils l6 and I1, thereby substantially completely cancelling or neutralizing the field. It is important to note also that when the strength of the ambientfield changes, the apparatus automatically maintains a completely neutralizing field. Suppose now by way of illustration that the ambient field suddenly decreased in value whereby a resultant field is set up at coils I f: and i? which has a direction opposite to that of the ambient field. The current through coil 22 decreases, thereby to reduce the resultant field substantially to zero and to set up a new condition of equilibrium in which the opposing field and the ambient field are substantially equal and opposite in polarity. The fact that the generated field seeks a point, i. e., that value where it is substantially equal to the ambient field, is an important feature of the stabilizing operation of the apparatus in one mode of operation thereof, as will be subsequently explained more fully.

Current indicating device 5, Figs. 1 and 3, may be calibrated to read the strength of the field generated by coil 22, in a manner well known to those skilled in the art, and hence may indicate the magnitude of the field which is measured by coils i and If. The direction of current flow through meter 5 will give an indication of the polarity of the generated field and hence of the polarity of the field which it was desired to measure.

Referring again particularly to Figs. 1 and 3, there is revealed a second bridge circuit comprising the aforementioned coils E i and E5 and the halves of transformer secondary 9. The coils are matched and substantially identical, and are adapted to be disposed within the magnetic field to be measured on either side of coils is and W, preferably equispaced therefrom with their aXes in alignment with each other. The coils Hi and is have balanced saturable magnetic cores It and II associated therewith. The coils IQ and I5, together with their respective cores, comprise magnetometer elements which be substantially identical to the magnetometer elements comprising coils 5E and I'l together with their respective cores. The center tap of the transformer secondary 9 and the point between coils I4 and I5 are connected to the input terminals of an amplifier 3 to be subsequently more fully described.

The coils I4 and is are each adapted to generate signals having characteristics determined by the polarity and strengths of the field along the cores thereof, when excited by oscillator presently to be described. Transformer I is designed to supply a saturating voltage of substantially sinusoidal waveform when excited by oscillator 4.

A circuit suitable for use in the oscillator d is shown in Fig. 3 and comprises the tubes Il l, I32, I52, and IE4. The oscillations are generated in tube IId. Whereas any suitable tube may be employed, tube II i is shown as a double triode having cathodes I I5 and H6, grids II! and H8 respectively, and anodes H9 and I respectively. The cathodes are connected together and to ground. Grid return to cathode from grid H1 is through resistance iZI; grid return to cathode from grid H8 is through resistance I22. Condenser l23 couples plate 526 to grid IIl; condenser i2 3 couples plate H9 to grid II 8. Potential is applied to anodes H9 and I20 through resistances I25 and 52% respectively, these resistances being connected together and thence through variable resistance III, dropping and decoupling resistor H2, and lead 32 to the aforementioned source of plate supply I97, decoupling condenser II3 being connected as shown from one end of resistance III to ground. Between the anodes II9 and I20 is connected the L-C tuned circuit comprising inductance l2? and condenser l28 in parallel. Suitable choice of component values being employed, the circuit thus traced constitutes a negative resistance oscillator of conventional design, the operation of which will be readily understood by those skilled in the art. The generated voltage of substantially sinusoidal waveform developed across the aforementioned L-C circuit is applied to the primary Hit of a coupling transformer generally designated at I29, which supplies its output to tube I32. The frequency of oscillations may be 800 cycles per sec.

The circuit of tube I32 comprises a push-pull amplifier of conventional design. Whereas any suitable tube may be employed, the tube shown is a double triode having cathodes I33 and I34, grids I35 and I 35 respectively, and anodes I3! and 38 respectively. The ends of the centertapped secondary I35 of transformer I29 are connected to grids I3 and l35 respectively. Cathodes 233 and IM are connected through biasing resistors I35 and ground and to the center tap of secondary IBI. The anodes I3? and I 38 are connected to the ends of the center-tapped primary I43 of a coupling transformer I 52, the center tap of the primary being by-passed by capacitor IM to ground and connected through resistors t ll and H58 and lead 32 to the plate battery fill. The operation of the amplifier circuit is conventional and the amplifier may be biased for class A operation if desired to secure optimum fidelity of the output. It is noted that bias resistors I39 and Ili are not by-passed, thereby providing some degeneration. It is further noted that the cathodes I33 and I3 3 are connected through R-C networks to the plates of the succeeding stage of amplification, thereby providing additional degeneration in a manner well known to those skilled in the art.

The final stage of push-pull amplification comprises the pentode tubes I52 and I64 having cathodes I 53 and IE5 respectively, control grids I54 and I 55 respectively, screen grids I55 and I ii? respectively, suppressor grids I56 and I68 respectively, and anodes E51 and IE9 respectively. The ends of the center-tapped secondary hi l of coupling transformer It? are connected to grids we and 65 respectively, the center tap of the secondary I 35 being connected to ground, and through biasing resistance I 39 to the oathodes 153 and 565 which are connected together. The suppressor grids are tied to the respective cathodes. The anodes I5? and I69 are connected to the ends of the center-tapped primary IlI of a coupling transformer generally designated at Hi), the center tap of the primary being connected to both the screen grids I55 and I5? and thence by lead Iii) to the center tap of primary I53. This point is, as aforedescribed, at a suitable plate potential. The operation of the amplifier circuit thus traced is conventional. Anode I57 of tube IE2 is connected through condenser IEG and resistance M5 to cathode I33 of tube I32; anode I69 of tube its is connected through condenser ifii and resistance I46 to cathode its of tube I32. These connections provide for degeneration and a resulting improvement in the waveform and fidelity of amplification, in a manner well known to those skilled in the art. The secondary I72 of trans- It respectively to 13 former I10 is connected to the primary 8 of the aforementioned output transformer I, which has the center-tapped secondary 9 thereof connected in the aforementioned bridge detector circuit with the gradiometer detector coils I4 and I5.

The bridge of coils I4 and I5 is designed to measure the field gradient existing between the coils thereof. Hence, the coils are connectd so that the detector or bridge output is proportional to the difference in field strengths at the two coils. It will be apparent from the foregoing discussions of the relations existing between harmonics, the direction of the axial field along the core, and the polarity of the exciting voltage, that such an output may be obtained under a condition in which the coils are wound in the same direction and the axial fields are of the same polarity.

Assume now by way of description that the coils l4 and I5 are wound in the same direction and no axial field is present. The voltages in the two coils are symmetrical and add algebraically to zero in the bridge output. The curves and P of Fig. 14 represent the induced voltages in coils I4 and I5 respectively and illustrate this condition of operation. Assume now that the coils are disposed within a uniform magnetic field having no gradient therebetween. The axial components along the cores III and II will be of the same polarity. During the half cycle when the axial fields aid the exciting voltages, the peaks of the voltages in both coils are advanced in time by equal amounts, and increased in amplitude by equal amounts. Since the changes are equal, the voltages on either side of the zero axis remain symmetrical, and add algebraically to zero. During the next half cycle when the axial fields oppose the exciting voltages, the peak voltages of the coils are reduced by equal amounts and retarded in time by equal amounts. The waveforms, however, remain symmetrical with respect to the zero axis, and add algebraically to zero. If perfect balance existed, the detector output would be zero when the coils I 4 and I5 were disposed within a uniform magnetic field. However, because of inherent unbalance conditions, some output does exist even in the absence of a field gradient. A typical output waveform is that shown in Fig. 6, in which the alternations are symmetrical with respect to the zero axis.

Assume now by way of description that the coils I4 and I5 are disposed within a field having a gradient therebetween. Fig. 14, to which particular reference is now made, illustrates the resulting distortions of the voltage waveforms when the strength of the field at coil I4 exceeds the strength of the field at coil I5. The peak voltage 0 in coil I I is set forward from the time reference line x by an amount a and increased in amplitude an amount 0 resulting in wave 0, whereas the peak of voltage P in coil I5 is set forward a lesser amount I) and increased in amplitude by a lesser amount d resulting in wave P. On the next alternation, waves 0' and P are retarded and reduced in amplitude, wave P being retarded and reduced by correspondingly lesser amounts. The result is that the waveforms O and P are unsymmetrical with respect to the zero axis and do not add algebraically to zero. There is a major second harmonic component (as well as other even harmonics), which renders the output signal unsymmetrical with respect to the zero axis, and in which alternations of one polarity exceed in amplitude the alternations of opposite polarity. In a typical case, the detector output resembles the waveform of Fig. 7. The difference in peak amplitudes is proportional to the difference in field strength at the two coils.

Assume now by way of description that the field changes so that it is of greater magnitude at coil I5 than at coil I4, and in an amount substantially equal to that by which the field at coil I4 formerly exceeded that at coil I5. The resulting waveforms, when added algebraically, give a detector output voltage resembling the curve of Fig. 8, in which negative alternations exceed positive alternations in peak amplitudes in amounts proportional to the difference in field strength at the two coils.

The detector output may be fed to an amplifier generally designated by the numeral 3 in Fig. 1, in which the alternations positive and negative with respect to the zero axis are separately amplified and ultimately combined in a circuit which will produce a magnitude which varies proportional to the peak differences of the alternations and in directions determined by which alternations exceed the others in amplitude, suitable indicating means being provided.

An amplifier circuit suitable for use in the amplifier 3, is shown in Fig. 3 and comprises the tubes I85, 291, MI, and 25L The amplifier input transformer I15 has a primary I16 with condenser I14 connected thereacross, and has a center-tapped secondary I'I'I. Whereas any suitable tube may be employed in the first amplifier stage, the tube I35 shown is a double triode having cathodes I35 and I81, grids I88 and I89 respectively, and anodes I90 and I9I respectively. The center tap of the secondary I'I'I is connected to ground; the ends of the secondary are connected to tapped voltage dividers I93 and I 94 having the other ends thereof connected to ground. The taps of the voltage dividers I 93 and I94 are connected to the contacts of switches I95 and IE8 respectively, the arms of the switches being connected to grids I88 and I89 respectively. The switches and tapped dividers provide a step gain control of conventional design. The cathodes I and I81 are connected together, and through biasing resistor I 92 to ground and to the center tap of secondary Ill. The anodes I and I9I are connected to the ends of a centertapped primary 292 of interstage coupling transformer 2M, the center tap of the primary having by-pass and decoupling condenser 205 connected therefrom to ground, and being connected through dropping and decoupling resistor 208 and conductor 32 to plate supply battery I01. The operation of the amplifier circuit is conventional.

The transformer 2M has two matched secondaries 293 and 284 which deliver their outputs to the next tube. Whereas any suitable rectifier tube may be employed, the tube 201 is a double diode having cathodes 208 and 209, and anodes 2H] and 2H respectively. secondaries 203 and 284 are connected to anodes 2I0 and 2H respectively, whereas the other ends of the secondaries are connected through resistors 2I2 and 2I3 respectively to the cathodes 208 and 209, which are connected together. Resistor 2I2 has connected thereacross condenser 2I'I while resistor 2 I3 has connected thereacross condenser 2I8. The circuit thus described constitutes a dual rectifier in which a rectified voltage having a value corresponding to the peak value of the A. C. voltage of one half cycle is deit? veloped across resistor 212 and condenser 2H, while a rectified voltage having a value corresponding to the peak value of the A. C. voltage of the other half cycle is developed across resistor 2|3 and condenser 21a. The ends of the resistors 2l2 and 213 adjacent the transformer secondary are connected to the grids 224 and 225 respectively of a double triode tube 22! having cathodes 222 and 5:23 respectively, and anodes 226 and 221 respectively. The cathodes 222 and 223 are connected together and through biasing resistor 2H5 to ground. The grids 2 i and 225 are connected through resistors 2M and 2:5 in series having the center point therebetween connected to ground. The condenser 259 is connected across the grids 22d and 225 as shown to provide a suitable time constant for the circuit. The circuit just described constitutes a differential peak amplifier. The voltage across resistor 2l2 tends to make grid 22d negative and grid 22% positive, whereas the voltage developed across resistor 2l3 tends to make grid 225 negative and grid 22'; positive. When the rectified voltages are equal they cancel and no resulting voltage is applied to the grids. When the rectified voltage across one of the resistors exceeds that across the other, for example, that across 212 exceeds that across resistance 213, grid 225 is made more positive and grid 22% more negative, resulting in an increase in the current in the anode 22'! and a decrease in the current through anode 226. In this respect, the operation of the circuit is similar to that of amplifier i. The anodes 225 and 22? have connected thereacross resistors 23d and 23I in series, with the center point between resistors being connected through resistor 229 and lead 32 to the aforementioned plate supply battery H17, and with the decoupling condenser 228 connected from resistor 22: to ground, as shown. An indicating circuit comprising meter 6 in series with resistor 233 is connected across the anodes 225 and 221 as shown, the meter having a shunt resistance 23% connected thereacross, and the resistance 233 having a cut-out switch 235 connected thereacross. The condenser 235 is also connected across the aforementioned anodes to provide a suitable time constant for the circuit.

The meter 6 may be of the type in which zero reading occurs in the center of the scale. When equal currents are flowing through anodes 226 and 227, the voltage drops across equal resistors 239 and 23! will be equal and of opposite polarity with respect to the meter 5, and no current will flow therein. Should one of the grids 22s or 225 become more positive than the other as a result of rectified peak voltages of different amplitudesbeing developed across resistors 212 and 2l3, the current through one anode will exceed the other, and a voltage will be developed across meter 6, of a magnitude proportional to the difference in rectified peak amplitudes, and of a polarity depending upon which rectified peak Voltage exceeds the other. Meter 5 accordingly indicates the field gradient at coils id and i5, and the direction of the gradient.

An additional stage of amplification is provided to permit the use of a recording meter Bil if desired, and comprises double tricde tube 25! having cathodes 252 and 253, grids 254 and 255 respectively, and anodes 256 and 25? respectively. Coupling condensers 231 and 238 couple grids 254 and 255 to anodes 225 and 221 respectively. The cathodes 252 and253 are connected together and are connected through biasing resistor 2% to ground. The grids 254 and 255 are connected through resistors 243 and its in series, the resistors having their midpoint connected to ground and to biasing resistor 24d. Resistor 243 has condenser 239 connected thereacross, while resistor 245 has condenser 249 connected thereacross. A further connection is provided between the cathodes through coupling condenser 2% and lead 109 to the secondary of the aforementioned transformer dl for purposes to be hereafter described. The anodes 255 and 251 have connected thereacross the aforementioned recording and indicating meter at, and have connected thereacross also a resistance network comprising in series resistance potentiometer 269, and resistance 259 having condenser 263 connected thereacross. The arm of the potentiometer, which serves as a balancing device, is connected to the decoupling condenser 255, and through decoupling resistance 25? and lead 32 to the plate battery [67. The anodes 256 and 257 have connected thereacross an output or utilization circuit comprising condenser 254 in series with headphones 265, the headphones having shunted thereacross an L-C network comprising condenser 262 and inductor 26E. Headphones 265 are used as an aural indication of gradient in cases where that type of indication is desired. In addition to the D. C. potential which appears across resistances 25d and H5, there is an A. C. component which is ripple caused by the slight discharge of condenser 21'! and 2l3 between peaks. This A. C. goes through an amplitude minimum at zero peak difference of alternate half cycles of the rectified A. C. voltage and may be used as an indication of zero gradient. The headphones are not normally phase sensitive. Accordingly, no polarity indication is given thereby. One phone, however, could be arranged to be responsive to one polarity of the A. C. ripple and the other phone responsive to the opposite polarity thereof. The tuned circuit 252 and 266 enhances the A. C. ripple current in the headphones.

Due to the blocking action of coupling condensers 23? and 238, tube 25! is responsive only to changes in voltage and meter 60 thus indicates changes in gradient only. This is quite useful when the magnetometer gradiometer is required to indicate low frequency signals such, for example, as when it is towed past a mine and it is desired to eliminate the efiect of instrument drift and slow changes in the ambient gradient. It will readily be understood that indications are obtained on meter 6 proportional to the difference in the peak amplitudes of the half cycles of voltage exciting transformer H5. The meter 69 may be of the recording type in which the zero position occurs in the center of a moving chart if desired, since the current flows through this meter in difierent directions selectively in accordance with which half cycle at primary Ht has the larger peak amplitude.

Assume now by way of description that the complete circuit of Fig. 3 is in operation, and that the apparatus is disposed within a field having a gradient between the two outer coils J4 and [5. The average field coils l6 and I! generate a signal proportional to the magnitude of the field at the center of the instrument, or of the average field. From this signal a direct current is derived and applied to coils 20, 22, and

17 2|, which set up fields opposing the field to be measured.

Referring now to Figs. 9 and 10, which illustrate the operation of the system under one condition which may be encountered in service, the points designated 348, 349, and 350 designate the approximate positions in space, or magnetic centers, of the gradiometer coil l4, bridge coils I6 and I1, and gradiometer coil I5 respectively. These points are in substantial alignment as previously explained and as shown in Fig. 9. They are also, in the preferred embodiment, substantially equidistant; arrows C, E, and F respectively are of different lengths to indicate the relative strengths of the component of the field acting on the coils at the three positions. In Fig. 10, neglecting for the moment the field set up by coils 29 and 2|, the sloping line A illustrates the gradient of the field to be measured between points 348 and 350, which, as afore mentioned, correspond to the points in space at which coils l4 and I5 are effectively located. It will be understood that the slope of the line is exaggerated for purposes of illustration; in actuality, the apparatus of the subject invention is capable of measuring gradients of 0.5 gamma per foot upon a background or average field of 50,000 gamma, in which case the slope of the line A would be very gradual. The arrows C, E, and F as before stated represent the relative magnitudes of the components of the ambient field, except that these components are actually in alignment whereas Fig. 10, for purposes of illustration, shows them as parallel.

The number of turns which are wound upon the end nulling or neutralizing coils 20 and 2| may be somewhat less, or slightly greater than, the number wound on the center coil 22, depending upon the manner in which the apparatus is desired to operate. If it is desired to generate fields at the gradiometer coils which are substantially equal in magnitude to the field to be measured at the center, a few more turns, of the order of 1.0 percent, may be wound on the end coils 29 and 2| than on the center coil 22. If it is desired to generate fields at the end coils 20 and 2| of somewhat smaller magnitude than the average intensity of the field to be measured, the end coils 20 and 2| may have fewer turns than the center coil.

Fig. 10 illustrates the operation of the system under a first mode of operation in which the last described condition exists. The arrows X, Z, and Y represent the fields set up by the coils 20, 22, and 2| respectively, and are opposite in direction to arrows C, E, and F respectively. At the two outer points, the resultant fields are indicated by the arrows V and W respectively. It is noted that these resultant fields are of the same polarity. Under these conditions, the curves and P of Fig. 14 illustrate the voltages developed in the two coils I 4 and I5. The output of the detector resembles the waveform of Fig. 7, and the meter 6 of amplifier 3 gives an indication proportional to the difference in field strengths at coils I4 and I5, and the direction of the deflection of the meter indicates the direction of the gradient indicated by the arrows of Fig. 9.

Assume now by Way of description a second mode of operation in which the fields generated by the coils 20 and 2| are increased, as by adding additional turns thereto, so that the fields generated by the coils are equal to the distance between line K and the base line, Fig. 10. Since the resultant field at coil I is substantially zero,

the voltage generated in this coil is that which would be generated in zero external field, whereas the voltage in coil H is advanced and increased in amplitude, but not to as great an extent as in the first mode, since the resultant field at coil I4 or point 348 is less than formerly. When the even harmonic components of the voltages in coils I4 and I5 are added algebraically, the resultant will still be represented by the voltage of Fig. '1, and the difference in peak amplitudes of the alternations of the bridge output from coils I4 and I5 will still be proportional to the difference in field amplitudes, or to the field gradient.

Assume now by way of description a third mode of operation in which the number of turns on coils 20 and 2| are further increased until the fields generated thereby are substantially equal in magnitude to the field to be measured at the center point 349. Fig. 11 illustrates diagrammatically the field conditions which would then exist. Arrows D, L, and G represent the opposing fields set up by coils 20, 22, and 2| respectively. The resultant fields at coils I4 and I5 are indicated by arrows S and T respectively. It is noted that the resultant field at point 359 is now reversed in direction from the polarity of the field to be measured at this point, since coil 2| generates a field G greater than the original field F.

Fig. 13, to which reference is now made, is illustrative of the waveforms of the voltages in coils I4 and I5 under the conditions of the above described third mode of operation. It is observed that the peak voltage in one coil, coil I4 for example, is advanced in time and increased in amplitude, while the peak voltage in the other coil I5 is reduced and retarded. The bridge detector output will resemble the waveform of Fig. '7, and will, as before, be proportional to twice the magnitude of the resultant field at either coil, and hence will still be proportional to the field gradient existing between coils 4 and IS, the gradient in this case being the algebraic sum of the resultant fields at the coils. The meter 6 indicates the strength of the gradient, and also indicates the direction of the original field gradient.

Referring again to Fig. 3, there are disposed in predetermined coaxial relationship with coils I4 and IS a pair of supplementary coils I8 and I9 respectively, these being connected in magnetic opposition and in series with indicating meter I to the terminals of a double-pole doublethrow reversing switch I8I having connected across the center terminals thereof a variable voltage divider network comprising the battery I84, variable resistances I82 and I83, and resistance I19. This circuit permits a measured direct current of chosen polarity to fiow through coils I8 and I9, thereby to produce a small measured gradient for calibration purposes and zero adjustment.

Reference is made now to Fig. 4, which shows the assembly of detector, auxiliary nulling, and field generating coils, and the supporting framework therefor. A main cylindrical casing or housing 210 composed of non-magnetic material has threaded ends 2H and 212 having retaining rings 213 and 214 in threaded engagement therein respectively, the rings having bores or openings 26B and 269 therein respectively. The inner surfaces of the rings are characterized by annular recessed portions in which gaskets 215 and 215 are disposed respectively, these normally abutting against the shoulders 3G0 and 3|8 respectively of a pair of supporting members 299 and 3|! disposed within the cas ng 279. The member 299 has mounted therein a pair of bushings 292 and 294 for passage of lead wires to the coils inside the assembly. The supporting members 299 and 3|? maintain in position within the casing a pair of shock insulating cylindrical supports 29| and M9 respectively, composed of soft rubber or other suitable material. Supported inside the main casing 270 and maintained in position therein by the aforementioned shock mountings 29| and (H9 is a cylindrical casing 292 of smaller diameter and composed of non-magnetic material, for example, brass. The supports 29l and 319 also prevent deflections of the outer cylinder 27E] resulting from its own weight or other loads from being transmitted to the inner cylindrical casing 292. Disposed within the casing 292 at spaced intervals are three main coil supports 28l 363, and 322. The support 281 at the left end of the casing, Fig. 4, provides support for the field generating coil 2c, the calibrating coil l8,

and the detector coil 14, the support 28! to be equispaced on either side of the axis of the support, having the wall 3% therebetween and having smaller end portions 399 and 3 respectively forming shoulders 3H and 3 i2. Disposed within the bores 39'! and 323 are coils l6 and [1 respectively, having aforementioned cores l2 and I3, the cores {2 having end flange or rib supports 36% and 3&2 fixed thereon, the fiange 3M abutting against the shoulder 3i i, and the core I3 having end rib supports 33i and 332 fixed thereon, the rib 33! abutting against the shoulder 3l2. Disposed within an aperture 329 having a slanting wall is a locking piece 3 i 5 having a threaded bore therein for passage of the screw 3 IS. The screw passes through a small aperture 3M communieating with the larger aperture 328, and the end 3 it of the screw abuts against the wall of aperture 32%. Turning of the screw causes the lock- -ing piece EH5 to travel to a position where pressure between it and the inner wall of casing 292 locks the assembly in place.

The left end of casing 2952 has threads on the inside near the end thereof, in which is threaded a retaining ring 291, which abuts against a gasket 296, the pressure of which against an annular supporting piece or ring 225 maintains the last named member firmly in abutting relation to the main support 28!. It is observed that the body of the casing 292 has recessed portions on the inside thereof forming shoulders 34,! and 342. The inner end of support 253! abuts against shoulder 34 I, preventing movement of the support to a further position within the cylinder. The annular supporting member 295 has, passing through threaded bores therein, a pair of adjusting screws 28% and 295, for alignment purposes to be hereafter apparent. The cylindrical coil support 28! has a cylindrical recessed portion with shoulders 28! forming a coil cavity for coil 20. Disposed Within the inner cavity of the cylindrical support 28l is a tubular body 233 having the coil i8 Wound thereon and having a large head 285 and neck 23% attaching the head thereto, the head being adapted to fit snugly against the shoulder 286 formed by lip 282 of support 28l, the body 283 having a longitudinal bore 28!) therein for receiving the detector coil I 5, which has the aforementioned core iii, the core having fixed thereon ribs 278 and 219,. the inner end of the core passing into the small bore extension 217 and snugly fitting therein. The supporting body 283 has a reduced end portion 238 for receiving a tapered end cap 228. This cap is adapted to have pressure exerted thereon by the aforementioned adjusting screws 289 and 2963, the tension of the screws causing the body 283 to bend slightly at neck 28s, thereby to permit precise alignment of the coils axes with those of the other coils in casing 292.

The coil assembly of the right hand end of the cylinder 21!], Fig. 4, is substantially similar to the apparatus just described at the other end and need not be further described, the View of supporting member 344 being rotated degrees from that of annular supporting member 295.

It is understood that suitable lead wires will be brought to the various coils in any convenient manner to permit their connection as dis closed in Fig. 3.

In the subject invention, the gradient, or magnetic field gradient, is expressed by the ratio AH/AX, where AH is the difference in field intensity at the two gradiometers, and AX is the distance between the magnetic centers of the gradiometers. The subject apparatus is designed to measure extremely small gradients upon relatively large background fields. For that reason, balance considerations with respect to the detector coils are important. As previously stated, adjusting screws 289 and 296 are provided in the coil supporting assembly of Fig. 4 for precisely aligning the coils. If desired, trimmer coils may be employed, or variabl resistors may be added in parallel with the various, coils to assist in balancing. Certain design considerations are important. For high stability of the zero reading, preferably the magnetic core material should have very low retentivity and be driven fairly well into saturation each half cycle. The material should preferably be rather easily saturated and should have a steep BH char cteristic. As previously stated, a material known in the trade as Permalloy is suitable, in thin or laminated core form. Permalloy is characterized by high sensitivity to small changes in the magnetizing force. In the design of the coil and core, the coil should preferably be at least as long as the core, to avoid a condition in which the ends of the core are not carried sufiiciently far into saturation, thereby to avoid spurious zero readings.

The ambient field stabilization provided. by the opposing fields generated by coils 2t, 22, and 2|, reduces the various balance requirements. As before explained, in the condition of zero axial field and zero gradient, there is a signal in the output circuit of the gradient detector bridge which results from residual resistive, capacitative, and inductive unbalance of the bridge, this being represented by the curves of Fig. 6. This signal contains components of the fundamental oscillator frequency bilize in temperature.

and odd harmonics thereof. Even when extreme care is exercised in the manufacture of the gradient detector cores, their 3-H curves are not identical. The effect of a uniform ambient field is to shift the operating points on the two B-H curves, and thereby, because of diiferences in the sensitivities of the cores at the new points, to produce asymetry in the signal directly, that is, differences between peak amplitudes. ing a uniform field to the two slightly unbalanced cores is to produce a spurious difference between the positive and negative peaks of the signal in the bridge output, resulting in errors in measurement. This undesirable effect is greatly reduced by ambient field neutralization, in which the effective magnitude of the ambient field is substantially reduced to zero, thereby preventing large shifts in the operating points of the BH curves of the two cores with attendant changes in relative sensitivity.

It is important that the proportions of the field which are nulled at coils l4 and I5 be exactly equal, thereby to avoid the introduction of errors in the gradient measurements. To this end, coils and 2! are precisely balanced and aligned. The fields generated by coils 2G and 2| are, as before stated, controlled in magnitude by the magnetometer bridge of coils it and I1. generate a field at the magnetometer bridge opposing the field to be measured provides a system of negativefeedback. This arrangement, as will be understood by those skilled in the art, minimizes the requirements of the amplifier system to maintain calibration stability, and makes stability dependent largely upon the mechanical features of the coil 22 and magnetometer bridge. Since the current through coil 22 is also circulated through coils 20 and 2|, automatic field stabilization is provided at the gradient sensitive elements and the bridge output of the coils l4 and I5 provides a true measure of the gradient. However, as will be subsequently explained, in another embodiment of the invention coil 22 is omitted, the output of the centrally located magnetometer being sufficiently linear and stable over the range of fields to be measured to permit field stabilization at the gradient sensitive elements, variations in the average value of the ambient field, as seen by the two gradient sensitive coils l4 and i5, being minimized.

'In. the operation of the above described apparatus, the equipment is turned on and the tubes and components thereof allowed to sta- As aforementioned, a lead connection I09, Fig. 3, between oscillator 2 and amplifier 3 is provided whereby a 600 cycle voltage from the magnetometer oscillator 2 is applied through condenser 245 to the self bias resistor 244 of tube 25! of amplifier 3. This voltage is applied in phase on the grids 254 and 255 of the tube. Potentiometer 260 in the anode circuit of the tube is adjusted until the output meter 60 reads zero, or the tone is balanced out in headphones 255, in which condition the stage of amplification of tube 25I is balanced.

The apparatus may be calibrated by generating a field parallel to the axis of coils M and I5, having a known field gradient and having a known average value at coils it and i1, and observing the response of the gradient indicator 5; and average field indicator 5. It is contemplated Accordingly, the effect of app y-' It is noted that employing coil 22 to that, if desired, calibration circuits of any convenient design, in addition to the circuit of coils l8 and i9, may be included in the apparatus for injecting known direct currents into the various coil circuits for generating known fields. therein, or to apply simulated bridge output signals to amplifiers l and 3, to permit calibration of the device, in 'a manner which will be apparent to those skilled in the art.

Whereas it has been noted hereinbefore that the difference in peak amplitudes of the alternations of the bridge output voltages are proportional to twice the intensity of the average field, in the case of the magnetometer bridge of coils I 6 and I1, and to the gradient in the case of the gradiometer bridge of coils l4 and I5, it is understood that the meters 5 and 6 may be calibrated in field strength and gradient, as will be understood by those skilled in the art.

Whereas the frequencies of oscillations of generators 2 and 4 have been stated to be 600 and 800 cycles/sec. respectively, it is understood that any suitable frequencies could be employed. The same frequency may, if desired, be employed for both gradiometer and magnetometer bridge circuits, and a common oscillator may be used, sufiicient care being exercised in the isolation of these gradient and average field circuits.

Reference is made now to Fig. 5, which shows in block form an alternative physical arrangement of the detector coils. Instead of being disposed side by side as in Fig. 4, the magnetometer detector coils l6 and I! may be axially aligned and spaced apart a predetermined relatively small distance between gradiometer detector coils M and 15, with the axes of all the coils in substantial colinearity, coils I6 and I! being wound in suitable directions to permit operation asillustrated in Fig. 13 and a detector output signal resembling the waveforms of Figs. 7 and 8. The detector signal of coils I6 and I! would be representative of the strength and direction of the field at a point substantially midway between coils l6 and H. Coil 22 would be symmetrically disposed with respect to coils l6 and H, with its axis in alignment with. the axes of the detector coils.

In Figs. 1 and 3, the coil l4 and core ID, the coils l6 and H and cores I 2 and [3 respectively, and the coil [5 and core ll constitute three magnetometers for deriving three signals having characteristics representative of the magnitude and direction of the magnetic field to be measured at three points therein. These elements are of the fluxgate type, in which the signals comprise even harmonic components which are generated as the field to be measured aids or opposes the magnetizing force during alternate half cycles of the exciting voltage. Because of their adjacent positions and their connection in the aforedescribed bridge arrangement, coils l6 and i1 together may be thought of as constitut ing one magnetometer.

Reference is made now to Fig. 2, which shows a generalized form of the invention of Figs. 1 and 3. Three magnetometers 69, 5D, and 10 are positioned at preferably equally spaced intervals Within the field to be measured, all responsive to the same directional component of the field, and having sources of energizing potential 41, 2', and 48 associated therewith respectively. It is understood that any type of magnetometer which is adapted to deliver a signal having varying characteristics corresponding to variations in the strength and polarity of a magnetic field maybe 23 used for each of magnetometers. 59, 5d, and it. They may be of the type described heretofore, or

they may be of other types such, for example,

as that type in which two coaxial coils are employed about a saturable core which collects the lines of fiux. of the field to be measured. When such a core is driven to saturation periodically by a field set up by one of the coils, these lines are expurged from the core and cut the other or detector coil, inducing an output voltage therein proportional to the strength of the field.

Another example of a suitable type of magnetometer is the type well known in the art as the second harmonic fiuXgat-e which comprises a coil and core similar, for example, to coil Id and core Ii) of Fig. 1, and in which the core is periodically driven well into saturation whereby even harmonic frequency components are genera-ted in the coil in the presence of an external w field. The second harmonic component is selected, as by filtering, and amplified to provide a measure of the field component parallel to the axis of the core, the amplified output being proportional to the strength of the field component.

puts. The apparatus 49 may be, for example, a

two channel vacuum tube amplifier in which the signals from the magnetometers B9 and is are separately amplified and subsequently rectified to obtain direct currents, these being applied in polarity opposition to an output circuit including meter 6', the current in which will then have a magnitude depending upon the difference in the signals, and a direction depending upon which signal has the greater amplitude.

It may be desirable that apparatus 39 be designed to respond to the difierence of the magnetometer outputs before amplification, and to amplify the difference, and to produce from the amplified signal a direct current proportional to the gradient in the field. In the second harmonic type, it may be desirable to compare the outputs and select from the comparison product the second harmonic component fo amplification.

The magnetometer 50 energized from source 2' generates a signal which is amplified and recti- ,fied by the apparatus, which may be of conventional design, indicated by the numeral 1', and

applied to coils 20, 22, and 21 disposed in predetermined positions as explained previously, whereby upon energization they generate electromagnetic fields opposing the field to be measured. These opposing fields reduce :the background at the gradient sensitive magnetometers 69 and H3, and thereby increase the relative value of the gradient with respect to the average field.

Reference is made now to Fig. 16, which shows a slightly modified form of the circuit of Fig. 2 and in which fiuxgate elements comprising coils H and I5 and cores l0 and II respectively are employed in the magnetometers of the gradient sensitive circuit which is identical to the gradiome-ter bridge circuit of Fig. 1, and 'any suitable type of magnetometer 50 adapted to generate a :signal for neutralizing or automatic nulling pur '24 poses is disposed between the gradient sensitive elements and connected to supply he nulling signal thereto.

Reference is made now to Fig. 17, which shows a second embodiment of the invention, employing field stabilization, and in which no QDPOSing field is set up at the center magnetomete element. As before, the two end gradient sensitive coils l4 and I5 are disposed a predetermined distance apart within the field to be measured, with their respective saturable cores .ID' and H in substantial colinearity. These coils may be connected in a bridge circuit substantially similar to the arrangement of Fig. 1. A magnetometer generally designated in block form by the reference character 50 is disposed between coils l4 and i5 and adapted to be responsive to the same directional component of the field to which coils M and I5 are responsive. A common source of oscillations 2 of substantially sinusoidal waveform excites both magnetometer 50 and transformer I, having therebetween unidirectional coupling devices such as vacuum tube amplifiers 6'! and 68 respectively so that substantially no undesirable coupling is introduced between the electrical circuit of the center magnetometer and the circuit of the gradient sensitive coils. The magnetometer 50 delivers its output to the amplifier I having a variable attenuator 59 in the output thereof, and which delivers its energy to the pair of coil-s 20 and 2| which are, as before explained, adapted when energized to generate electromagnetic fields in alignment with the component of the field to be measured which lies alon the longitudinal axes of cores l0 and II.

In the operation of the device of Fig. 17, the coils 2i! and 2| generate fields opposing the field to be measured. Variable attenuator 59 is adjusted to provide the desired value of field strength. As the value of the generated field approaches the value of the field to be measured, the difference in the values representative of the gradient of the field becomes increasingly large by comparison with the remaining steady component of the field.

Reference is made now to Fig. 10, which is i1- lustrative of the operation of the circuit of Fig. 17 except that no central opposing field Z is generated. The gradient is represented as before by the line A, the slope of which is exaggerated for clarity of illustration. The lines 0, E, and F, as before, represent the relative strengths of the field to be measured at points 348, 3-49, and 350 respectively which, in this case, are occupied by coil l4, magnetometer 5B, and coil l5 respectively of Fig. 1'7. As before, the fields set up by coils 2B and 2i are represented by lines X and Y respectively and the resulting field-s at points 348 and 359 by lines V and W respectively. The magnitudes of the opposing fields X and Y are adjusted to desired values by attenuator 59. The operation of the circuits will be apparent from foregoing descriptions of the operation of the circuits of Figs. 1 and 3.

Reference is made now to Fig. 18, which shows an alternative form of the embodiment of Fig. 17 and in which the aforementioned three magnetometers B3, 56, and 10 of any convenient design, are employed to generate the three signals. If desired, a common oscillator 4 may excite the gradiometer magnetometers. At 49 is shown the aforementioned apparatus for comparing the outputs of the magnetometers 69 and 10.

Reference is made now to Fig. 15, which shows 25 a circuit diagram, partially in block form, of a magnetometer gradiometer employing no field stabilization, suitable for use in less critical applications which do not require high sensitivity. In Fig. 15, the amplifier-rectifier generally designated in block form by the numeral 3, may be identical with the differential peak amplifier circuit of Fig. 3 comprising tubes I85, 201, 22!, and 25!, and associated components. The operation of the gradient detector without stabilization will be apparent from foregoing descriptions of the apparatus of Fig. 3, and need not be described in detail.

The apparatus described herein may be adapted for the measurement of cross gradients AHx AHy Ay A2 etc. by orienting the sensitive axes of the gradient in a plane perpendicular to the base line of the instrument. The average field detectors may also be similarly oriented.

The terms field and gradient as employed herein refer to the particular components along the magnetic axis and to which the detectors are sensitive.

Certain modifications may be made, such for example, as in the apparatus of Fig. 17 wherein coils 20 and 2| may be dispensed with and the opposing fields may be set up by coils l4 and I5 themselves, the D. C. current from the amplifier I being caused to flow through the coils, suitable choke coils being interposed between the coils and the amplifier I to prevent the A. C. voltage exciting the coils from reaching the output circuit of the amplifier, and suitable blocking condensers being interposed between the coils I l and I5 and the amplifier 3 and transformer 1 for preventing'the direct current from amplifier I from getting into these devices.

Any suitable means may be provided for heating the heater elementsof the various vacuum tubes. Where a common source of plate supply is shown, it is of course understood that several sources could be used if needed to avoid undesirable coupling effects.

While the invention has been described with particularity with respect to certain embodiments thereof which give satisfactory results, it will be understood by those skilled in the art, after understanding the invention, that various changes and modifications may be made without departing from the invention as defined by the appended claims, and it is our intention, therefore, in the appended claims to cover all such changes and modifications.

Whereas the magnetometer which generates the signal from which the nulling fields are derived is shown as disposed intermediate the two gradient detectors, it is of course understood that this magnetometer may be disposed on either side of either of the gradient detectors, so long as it is responsive to the same directional component of the field to be measured.

The invention herein described and claimed may be manufactured andused by or for the Government of the United States-of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

' 1. In apparatus of the character disclosed for measuring the gradient of a magnetic field, a pair of field detectors positioned in predetermined spaced relationship in said field and each adapted to generate a voltage proportional to the magnitude at the detector of the same predetermined directional component of the magnetic field, a third field detector positioned intermediate said detectors and adapted to generate a voltage proportional to the magnitude of said component of the field at said third detector, means for amplifying the voltage generated by said third detector, field nulling means comprising a pair of coils disposed in predetermined positions with respect to said pair of detectors respectively, circuit means connecting said pair of coils with the output of said amplifying means thereby to energize said coils and set up electromagnetic fields for nulling a predetermined portion of the field to be measured in accordance with the magnitude of the voltage generated by said third detector, and means operatively connected to said pair of detectors and responsive to the voltages generated thereby for indicating the difference in the magnitudes of said component at each of said detectors when said portion of the field has been nulled.

2. In apparatus of the character disclosed for measuring the gradient of a magnetic field, threev amplifying means thereby to generate electro-' magnetic fields for nulling predetermined portions of the field to be measured at each of the detectors respectively in varying amounts in accordance with variations in said amplifier output, means for indicating the strength of the field generated by the coil positioned at the intermediate detector, and means operatively connected to the outer two detectors and responsive to the signals generated thereby for indicating the difference in the magnitudes of said component at said outer detectors respectively when said portions of the field to be measured have been nulled.

V 3. In apparatus of the character disclosed for measuring the gradient of a magnetic field, three field detectors positioned at predetermined spaced intervals in said field and adapted to be responsive to the same component thereof, a source of potential of substantially sinusoidal waveform, a first circuit for energizing the center one of said detectors from said source, a second circuit for energizing the outer two of said detectors from said source, each of said detectors being adapted to generate a signal voltage proportional-to the magnitude at the detector of said component, amplifying means energized by the signal voltage from the center one of said detectors, a pair of coils disposed in predetermined positions respectively adjacent said outer two detectors, said pair of coils being operatively connected to the output of said amplifying means and adapted to be energized therefrom whereby nulling electromagnetic' fields are'set up at the outer two detectors opposing the component of the field to be measured, and means operativelyconnected to the outer two of said detectors and responsive to the signal voltages generated thereby for indicating the difierence in the magnitudes of said component at said detectors when a portion of the field to be measured has been nulled.

4. In apparatus of the character disclosed for measuring the gradient of a magnetic field, three signal generating means for individually deriving three signals respectively proportional to the magnitudes of the same directional component of a magnetic field at three points disposed in predetermined spaced relation within said field, means controlled by the signal derived from one of said signal generating means for generating electromagnetic fields at the other two signal generating means opposing the component of the field to be measured thereby respectively whereby a portionof said field component is nulled at each of said other two field generating means, and means connected to theother two signal generating means'and responsive'to the signals derived therefrom for indicating the difierence in the magnitudes of said component at said other two detectors respectively when said portion of the component of the field has been nulled.

In apparatus of the character disclosed for measuring the gradient of a magnetic field, three field detectors positioned with their magnetic axes in alignment with the same component of the field and at predetermined spaced intervals therein, a first source of potential of substantially' sinusoidal waveform and predetermined frequency, a second source of potential of substantially sinusoidal waveform and of a frequency differing substantially from said predetermined frequency and having no low order harmonics corresponding to the low order harmonics of said potential of predetermined frequency, said first source being connected to the center one of said detectors for energizing the center detector, said second source being connected to the outer two of said detectors for energizing the outer detectors, each of said detectors being adapted to generate a signal voltage proportional to the magnitude at the detector of said component, amplifying means energized by the signal voltage from said center detector, 2. pair of coils disposed in predetermined positions respectively adjacent said outer detectors, said coils being connected in cirshit with said amplifying means and adapted to be energized therefrom thereby to set up electromagnetic fields for nulling a portion of said component of the field at each of the outer detectors, and means operatively connected to the, outer detectors and responsive to the, signal voltages generated thereby. for indicating the difference in the magnitudes of said component at saiddeteeters respectively when said portion of the component of thefield has been nulled.

6a In apparatus of the character disclosed for measuring the gradient of a magnetic field, a signal generator having av core of saturable magnetic material therein and adapted to be disposed within the field to be measured, alternating current means connected to said generator for periodically saturating said core, said generator, bein adapted to generate an output signal comprising distortions in the. waveform of said alternating current when said core is disposed within said field and saturated by said alternating current means, said distortions being proportional to the magnitude of the component of the field parallel to the magnetic axis of the core, means for amplifying the'output of said genera- 11 .71,, means for deriving a direct current from said amplifier means proportional to the strength of said component at the generator, a pair of field detectors having their magnetic axes in alignment with said component of the field and disposed on either side of said generator at predetermined spaced intervals therefrom, each of said detectors being adapted to generate a voltage proportional to the strength at the detector of said component of the field, a pair of nulling coils disposed in predetermined positions respectively adjacent said two detectors and adapted to be energized by said direct current whereby electromagnetic fields are set up for nulling a portion of the component of the field at each of the detectors variably in accordance with variations in the signal from said generator, and means operatively connected to said detectors and responsive to the voltages generated thereby for indicating the difierence in the magnitudes of said component at said detectors respectively when said portion of the component of the field has been nulled.

7. In a gradiometer of the character disclosed, three magnetometers, means for supporting said magnetometers at predetermined spaced intervals within the field to be measured and with their magnetic axes in alignment with the same directional component of the field, each of said magnetometers being adapted togenerate a signal having characteristics representative of the magnitude at'the magnetometer of said component of the field, means controlled by the signal from one of said magnetometers for generating at the other two magnetometers respectively electromagnetic fields of predetermined value opposing said component ofv the field thereby to null predetermined portions thereof at said other two magnetometers respectively, and means operatively connected to said other two magnetometers and responsive to the signals generated thereby for indicating the dilferencein the magnitudes of said component of the field at said last named magnetometers respectively when said portion of the field to be measured has been nulled.

8. In a gradiometer of the character disclosed, three magnetometers, means for maintaining said magnetometers at predetermined spaced intervals within the field to be measured and with their magnetic axes in alignment with the same directional component of the field, each of. said magetometers being adapted to generate a signal having characteristics representative of the magnitude at the magnetometer of said component of the field, nulling means controlled by thesignal from one of said magnetometers for generating at each of said three magnetometers nulling' electromagnetic fields opposing the component of the field to be measured and including means for automatically maintaining said generated fields at predetermined intensities proportional to the magnitude of said component of the field to be measured at the said one of the magnetometers, means for indicating the strength of the nulling field generated at said one of the magnetometers, and means operatively connected to the other two magnetometers and responsive to the signals generated thereby for indicating the difierence in the magnitudes of said component at said other magnetometers respectively when a portion of the field to be measured has been nulled.

9. A stabilized gradiometer for measuring the gradient of a magnetic field comprising, in combination, threedetectors adapted to be positioned at predetermined spaced intervals within the field to be measured and responsive to the same 

